Post History

The annotation processor must be compiled before it can be used. This is not a problem if you're using it as a dependency from Maven Central, for example. If you want to use a custom processor, you usually split your project into two parts.

Your project structure could look as follows:
```txt
/Project
  /Preprocessor
    /src/main/java/com/folder/project
      /archive
        Annotation.java
      /preprocessors
        AnnotationProcessor.java
    pom.xml
  /Program
    /src/main/java/com/folder/project/program
      App.java
    pom.xml
  pom.xml
```

There are three pom.xml files. One root file and two for the `Preprocessor` and `Program` modules. `Preprocessor` is a dependency for `Program`, so it can be compiled and used before program is compiled.

`Preprocessor` contains the annotation and the annotation preprocessor. `Program` is the example module.

<br />


`/pom.xml`

```xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.folder.project</groupId>
  <artifactId>Project</artifactId>
  <version>1.0</version>
  <packaging>pom</packaging>

  <name>Project</name>
  <url>http://maven.apache.org</url>
  <modules>
    <module>Preprocessor</module>
    <module>Program</module>
  </modules>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.release>8</maven.compiler.release>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-compiler-plugin</artifactId>
      <version>3.11.0</version>
    </dependency>
  </dependencies>
</project>
```

<br />

`/Preprocessor/pom.xml`

```xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>com.folder.project</groupId>
        <artifactId>Project</artifactId>
        <version>1.0</version>
    </parent>

    <artifactId>Preprocessor</artifactId>
    <packaging>jar</packaging>
</project>
```

<br />

`/Program/pom.xml`

```xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>com.folder.project</groupId>
        <artifactId>Project</artifactId>
        <version>1.0</version>
    </parent>

    <artifactId>Program</artifactId>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>com.folder.project</groupId>
            <artifactId>Preprocessor</artifactId>
            <version>1.0</version>
            <scope>compile</scope>
            <type>jar</type>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.11.0</version>
                <configuration>
                    <annotationProcessors>
                        <annotationProcessor>
                            com.folder.project.preprocessors.AnnotationProcessor
                        </annotationProcessor>
                    </annotationProcessors>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>
```

<br />

`/Preprocessor/src/main/java/com/folder/project/archive/Annotation.java`

```java
package com.folder.project.archive;

public @interface Annotation {
    String value();
}
```

<br />

`/Preprocessor/src/main/java/com/folder/project/preprocessors/AnnotationProcessor.java`

```java
package com.folder.project.preprocessors;

import com.folder.project.archive.Annotation;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic;
import java.util.Set;

@SupportedAnnotationTypes("com.folder.project.archive.Annotation")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class AnnotationProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        for (Element e: roundEnv.getElementsAnnotatedWith(Annotation.class)){
            Annotation a = e.getAnnotation(Annotation.class);
            processingEnv.getMessager().printMessage( Diagnostic.Kind.WARNING, "Annotation found. Type: "+a.value() );
        }
        return false;
    }
}
```

<br />

`/Program/src/main/java/com/folder/project/program/App.java`

```java
package com.folder.project.program;

import com.folder.project.archive.Annotation;

@Annotation("class")
public class App
{
    @Annotation("method")
    public static void main( String[] args )
    {
        @Annotation("local variable") int x = 1;
        System.out.println( "finished main" );
    }
}
```

<br />

You will see a warning with the text `Annotation found` when you compile the project. You can do this from root, which will automatically build the preprocessor module and then compile the program.
There are three annotations, but only two `Annotation found` warnings will appear. That's because local variable annotations are ignored.